The ballast in railway is a layer of broken stones, gravel, moorum, or any other granular material placed and packed below and around sleepers for distributing load from the sleepers to the formation. It provides drainage as well as longitudinal and lateral stability to the track. Different types of ballast materials and their specifications are discussed in this article.
Table of Contents
Functions of Ballast
The ballast in railway track serves the following functions:
- Provides a level and hard bed for the sleepers to rest on.
- Holds the sleepers in position during the passage of trains.
- Transfers and distributes load from the sleepers to a large area of the formation.
- Provides elasticity and resilience to the track for proper riding comfort.
- Provides the necessary resistance to the track for longitudinal and lateral stability.
- Provides effective drainage to the track.
- Provides an effective means of maintaining the level and alignment of the track.
Types of Ballast in Railway
The different types of ballast in railways are described in the following:
1. Sand Ballast
Sand ballast is used primarily for cast iron (CI) pots. It is also used with wooden and steel trough sleepers in areas where traffic density is very low. Coarse sand is preferred in comparison to fine sand. It has good drainage properties, but has the drawback of blowing off because of being light. It also causes excessive wear of the rail top and the moving parts of the rolling stock.
2. Moorum Ballast
The decomposition of laterite results in the formation of moorum. It is red, and sometimes yellow, in colour. The moorum ballast is normally used as the initial ballast in new constructions and also as sub-ballast. As it prevents water from percolating into the formation, it is also used as a blanketing material for black cotton soil.
3. Coal Ash or Cinder
This type of ballast is normally used in yards and sidings or as the initial ballast in new constructions since it is very cheap and easily available. It is harmful for steel sleepers and fittings because of its corrosive action.
4. Broken Stone Ballast
This type of ballast is used the most on railways. A good stone ballast is generally procured from hard stones such as granite, quartzite, and hard trap. The quality of stone should be such that neither is it porous nor does it flake off due to the vagaries of weather. Good quality hard stone is normally used for high-speed tracks. This type of ballast works out to be economical in the long run.
Other Types of Ballast in Railway
There are other types of ballast also such as the brickbat ballast, gravel ballast, kankar stone ballast, and even earth ballast. These types of ballast in railway are used only in special circumstances.
Sizes of Ballast in Railway
Previously, 50-mm (2″) ballasts were specified for flat bottom sleepers such as concrete and wooden sleepers and 40-mm (1.5″) ballasts were specified for metal sleepers such as CST-9 and trough sleepers. Now, to ensure uniformity, 50-mm (2″) ballasts have been adopted universally for all type of sleepers.
As far as points and crossings are concerned, these are subjected to heavy blows of moving loads and are maintained to a higher degree of precision. A small sized, 25-mm (1″) ballast is, therefore, preferable because of its fineness for slight adjustments, better compaction, and increased frictional area of the ballast.
Requirements of a Good Ballast
Ballast material should possess the following properties:
- It should be tough and wear resistant.
- It should be hard so that it does not get crushed under the moving loads.
- It should be generally cubical with sharp edges.
- It should be non-porous and should not absorb water.
- It should resist both attrition and abrasion.
- It should be durable and should not get pulverized or disintegrated under adverse weather conditions.
- It should allow for good drainage of water.
- It should be cheap and economical.
Laboratory Tests for Physical Properties of Ballast
The following tests are recommended to judge the suitability of the ballast in railway track:
1. Aggregate Abrasion Value
To check for aggregate abrasion, a test sample of 10 kg of clean ballast conforming to the following grading is taken
Passing the 50-mm sieve and retained on the 40-mm square mesh sieve: 5000 g
Passing the 40-mm and retained on the 25-mm square mesh sieve: 5000 g
The sample, along with the abrasive charge, is placed in the Los Angeles machine, which is rotated at a speed of 30–33 rpm for 1000 revolutions. The sample is sieved and material coarser than the 1.70-mm sieve is washed, dried, and weighed. The difference between the original weight (A) and the final weight of the sample (B) is expressed as a percentage of the original weight of the test sample. This value is reported as the abrasion value.
Aggregate Abrasion Value = (A – B)/A
2. Aggregate Impact Value
To check for aggregate impact, the test sample is prepared out of the track ballast in such a way that it has a grading that passes the 12.5-mm sieve and is retained on the 10-mm sieve. The ballast sample is oven dried and placed duly tamped in the different stages in a cylindrical metal container with 75 mm diameter and 50 mm depth (weight A).
The cup of the impact testing machine is fixed firmly in position on the base of the machine and entire test sample is placed in it and compacted by 25 strokes of the tamping rod. The test hammer weighing about 14 kg is raised 380 mm above the upper surface of the cup and dropped. The test sample is subjected to a total of 15 such blows. The sample is then removed and sieved using a 2.36-mm sieve and the weight of quantity retained is measured (weight B):
Aggregate impact value = (A – B)/A
3. Flakiness Index
The flakiness index of an aggregate is the percentage by weight of the particles with a least dimension (thickness) less than three-fifths of their mean dimension. The test is not applicable to sizes smaller than 6.3 mm. Track ballast sample of sufficient quantity is taken to provide a minimum of 200 pieces, which is weighed (weight A).
The sample consisting of aggregates is sieved as per the prescribed procedure in a series of sieves. The flaky material is separated and weighed (weight B). The flakiness index is then determined by the total weight of the material passing the various sieves, expressed as a percentage of the total weight of the sample gauged.
Flakiness index = (B/A) x 100
4. Specific Gravity and Water Absorption Test
A sample consisting of at least 2000 g of aggregate is washed thoroughly to remove finer particles and dust. The whole material is then drained, placed in a wire basket, and immersed in distilled water at a temperature between 22°C and 32°C. The sample is shaken, jolted, and dried as per specific procedure. The sample is finally placed in an oven in a shallow tray at a temperature of 100°C to 110°C. It is then removed from the oven, cooled in the container, and weighed (weight C). The specific gravity and water absorption is calculated as follows:
Specific gravity = C/(B – A)
Water absorption (% by weight) = 100(B – C)/C
where A = weight in grams of saturated aggregate in water, B = weight in grams of saturated dry aggregate in air, and C = weight in grams of oven-dried aggregate in air.
Article Courtesy: Railway Engineering – Satish Chandra
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